fix return erro codes

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1681 c368ceb8-fd28-0410-a457-e46ee1d7cd4a
allow these 2 arguments to be NULL

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1682 c368ceb8-fd28-0410-a457-e46ee1d7cd4a